Multiple access communication systems are well understood in the art. Multiple access communication systems are designed to provide access to limited communication resources for a plurality of communication units for the purpose of transmitting communication messages, referred to as packets. The access methodology, referred to as a multiple access protocol, is chosen such that some appropriate set of performance constraints are met. Typical performance constraints include efficiency of communication resource use, communication message delay, and other similar factors. Multiple access protocols can be regarded as belonging to one of two general types, contention and non-contention.
Non-contention protocols are designed such that a communication unit desiring to send a packet is permitted exclusive use of a communication resource. One example of this type of protocol is time-division multiple access (TDMA) where the communication resource is divided into a plurality of time frames that are further subdivided into a plurality of time slots and each communication unit is assigned exclusive use of one or more time slots in each time frame.
Contention protocols are characterized by communication units that actively compete with each other to gain access to the communication resource. The slotted ALOHA protocol is an example of this type of protocol. In slotted ALOHA, a communication resource is divided into a plurality of time slots. A communication unit desiring to send a packet may transmit in a first subsequent time slot, taking care not to transmit outside of the boundaries of that time slot. If no other communication unit transmitted in that time slot, the packet transmission is considered successful. If, however, one or more other communication units transmitted a packet in the same time slot, the transmission will usually fail due to collision.
Reservation protocols, a sub-class of contention protocols, are also known. Reservation protocols attempt to combine certain aspects of contention and non-contention protocols to provide improved performance for a wider variety of communication system conditions. In one particular reservation protocol, the Reservation-ALOHA (R-ALOHA) protocol, like TDMA, the communication resource is divided into time frames that are further divided into time slots. When the communication resource is unused, the protocol operates similarly to slotted ALOHA. When a communication unit desires to send a packet, it transmits in one of the unused time slots, referred to as a random access slot. If the transmission is successful, i.e., it does not collide with another transmission, the communication unit is permitted exclusive use of the same time slot in subsequent time frames, referred to as reserved access slots, until the packet is completely transmitted. Thus the initial ALOHA transmission results in a subsequent reservation of a communication resource. Note that some method of feedback to the communication units regarding the success or failure of initial ALOHA transmissions is necessary in order for this protocol to be effective.
R-ALOHA is quite efficient for communication systems accommodating a wide variety of packet frequencies and sizes. Note, however, that when the communication resource in an R-ALOHA system is in the reserved state, the transmission of random access packets is inhibited for all communication units desiring to access the resource. Random access packets are held pending the release of the resource. Once the resource returns to the random access state, the units may transmit any pending random access packets.
If all the waiting units access the resource as soon as the resource becomes available, the packets will all collide, with no transmission being successful. It is therefore generally beneficial to not have all the units with pending random access packets transmit as soon as the resource is available. One method that may be used to reduce the likelihood of collisions is to have all units wait a random time after the channel becomes available before transmitting. This method contributes to higher overall delays in the delivery of the packets. Furthermore, the delays incurred are not necessarily fairly distributed. That is, a unit that had a packet it wished to send early on during the time the resource was in a reserved state might have to wait for a long random delay after the resource becomes available, while a packet from another later unit might have little or no delay, based on the random delays chosen. Access to the resource does not occur on a first-come, first-served basis. The overall effect is a large variation in the delivery delays of packets, with the relative delays experienced by different communication units being unrelated to the actual order in which their packets were generated.
Accordingly, a need exists for a multiple access method that provides increased utilization of a communication resource by a plurality of communication units with widely varying communication requirements, and that provides a more fair ordering of access to the communication resource by the plurality of communication units.